1. Field of the Invention
This invention relates to Accelerator Mass Spectrometry wherein stable and radioactive isotopes of an element can be successively designated for measurement using a high speed isotope selector. The process allows precision isotopic ratios between stable and rare isotopes to be measured continuously. Although limitations in scope are not intended, this invention has particular relevance to the fields of nuclear waste disposal and the detection of clandestine nuclear reactor operations, the detection of trace elements in medicine and cosmogenic age determinations.
2. Description of the Prior Art
The principles of Accelerator Mass Spectrometry (AMS) have been described in detail by a number of authors who have presented ways in which AMS technique can be applied to the detection of very rare isotopes, such as .sup.10 B, .sup.14 C, .sup.26 Al, .sup.36 Cl, etc. Such descriptions include, for example, U.S. Pat. No. 4,037,100 to K. H Purser; Purser, K. H., Litherland, A. E., and Gove, H. E. "Ultra-sensitive particle identification systems based upon electrostatic accelerators", Nuclear Instruments and Methods volume 162, page 637 (1979); and Elmore, D. and Phillips, F. M., "Accelerator Mass Spectrometry", Science volume 236, page 543 (1987) For many rare nuclei it is routine for the detection limits to be between 10.sup.-12 and 10.sup.-16 compared to the concentration of the elemental stable isotopes. These ratios indicate as much as six orders of magnitude greater sensitivity than is possible using conventional mass spectrometry.
Measurement of isotopic ratios
In order to determine isotopic ratios accurately it is desirable that at least one stable isotope of an element be measured at the high energy end of the accelerator simultaneously with the rare isotope. The importance of simultaneous measurements is that both isotopes are expected to have identical trajectories between ion source and the exit from the accelerator, and, thus, the efficiency of transmission through the AMS system will be comparable for both isotopes. Using such a procedure, transmission losses within the accelerator will be identical in first order, allowing isotopic ratios to be measured directly, and fractionation changes with time can be identified.
Accelerator limitations
While intense beam currents of the stable isotopes from the ion source are desirable for maximizing the detection rates for the rare species, most tandem accelerators are neither capable of continuously handling intense beams of more than .about.10 microamperes or of accepting the large intensity fluctuations inherent in slow switching between rare and stable isotopes. Thus, it becomes necessary to attenuate the stable isotope beams by a definite fraction or alternatively to inject ions of the stable isotopes in very short pulses so that the natural electrical capacities of the accelerator maintain the internal voltages more or less constant.
Mass switching
Referring to FIG. 1, it can be seen that in a typical AMS injector the negative ions from the source 2 are energy analyzed using an electrostatic deflector 1, following which a suitable magnetic field carries out mass analysis by directing the wanted mass ions through the defining slits 6, discarding all others. For such a layout and using particles having a charge state of 1.sup.-, the selected ions satisfy the equation:
M.multidot.E=K
where K is a constant which includes the magnetic field and the mass analyzer geometry, M is the mass of the selected ions and E is their kinetic energy. Thus, for a fixed magnetic field (i.e. K remains constant) the selected mass is given by: EQU M=K/E
Clearly, a range of masses can be selected at fixed field by modifying appropriately the energy of the ions within the magnetic field.
Referring again to FIG. 1 it can be seen that by applying appropriate voltages to the insulated magnet vacuum chamber 4 it is possible to change the energy of the ions locally within the magnetic field and select individual masses for injection without the necessity of rapidly changing the magnetic field--a procedure that eddy currents within the magnets within the magnetic steel can make difficult or even impossible. For Carbon-14, having an energy of 20 keV, the box voltages needed for selecting specific isotopes from the trio, .sup.12 C, .sup.13 C, .sup.14 C, are 3.33 kV, 1.54 kV and zero, respectively. Clearly, using the above mass selection scheme, only one isotope can be injected into the accelerator at a given instance, and it becomes necessary to time-share the accelerator with the rare isotopes being given more analysis time than the abundant isotopes.
Two issues must be carefully addressed to achieve precision ratio measurements: (i) The electric waveform applied to the vacuum chamber must be flat-topped and free from any overshoot and ringing that would cause the select mass beam to be partially intercepted as it passes through the mass defining aperture. (ii) To facilitate the calculation of isotopic ratios it is essential to know precisely the relative duration that each isotope is directed into the accelerator. For state of the art measurements this ratio should be known to about 1/1000.
Accelerator injection energies
While for some AMS systems 20 keV is a satisfactory injection energy, for the larger and older tandems where the optical admittance is smaller, higher injection energies are more appropriate and there are some systems operating at injection energies as high as 130 keV. Such elevated energies introduce practical difficulties for high speed mass switching, however, because, although the particle transmission of the spectrometer is improved, the switching voltages needed for mass selection increase with injection energy. As an example, at injection energies of 130 keV, switching between the carbon isotopes requires 21.6 kV for shifting from mass 14 to mass 12 and 10.0 kV to change from mass 14 to mass 13. In the important case of measuring concentrations of the long-lived .sup.36 Cl isotope, the situation is somewhat less demanding: 7.4 kV for shifting from mass 37 to mass 35, and 3.6 kV for shifting from mass 37 to mass 36.